(1) Field of the Invention
The present invention relates to a process for synthesizing a high-silica aluminosilicate molecular sieve which can be preferably used as a hydrocarbon adsorbent of exhaust gas purification system.
(2) Description of Related Art
In order for an exhaust gas purification system for an automobile or the like to be able to exhibit its catalytic activity, the catalyst used therein must be heated to at least its lowest operating temperature by, for example, the heat of exhaust gas. Therefore, when the temperature of exhaust gas is low as seen in the cold start of engine, the harmful components (e.g. HC, CO and NOx) of exhaust gas are hardly purified. HC, in particular, is generated in a large amount during the cold start and its purification is an important task.
In order to improve the efficiency of HC purification during the cold start, there have heretofore been known techniques of using, as a HC adsorbent, a molecular sieve made of a crystalline aluminosilicate (e.g. zeolite) and allowing the adsorbent to adsorb HC until the exhaust gas purification system used reaches its operating temperature.
For example, Japanese Patent Application Laid-Open No. 75327/1990 discloses an exhaust gas purification system for an automobile, using a Y type zeolite or mordenite as an HC adsorbent. Japanese Patent Application Laid-Open No. 293519/1992 proposes use of an adsorbent obtained by subjecting a H.sup.+ type ZSM-5 zeolite to ion exchange with Cu and Pd, in order to alleviate the effect of water adsorption, improve HC adsorbability and widen the temperature range of adsorption. Japanese Patent Application Laid-Open No. 63392/1994 proposes, for the same purpose, use of an H-, Cu- or Pd-ion exchanged pentasil type metallosilicate as an adsorbent.
With respect to the above pentasil (pentasil is a generic name for structures similar to ZSM-5) type zeolite, a relatively high-silica product can be synthesized easily and even a product having a Si/Al molar ratio of substantially infinity (containing no Al) can be synthesized by using a template.
However, direct synthesis of pentasil type zeolite of high Si/Al molar ratio (&gt;50) by using no template has heretofore been difficult.
Meanwhile, .beta. type zeolite and mordenite type zeolite are known to have large lattice strain when containing no Al, as compared with pentasil type zeolite. Therefore, in synthesizing .beta. type zeolite and mordenite type zeolite, it has been necessary to add Al (which has an ionic radius different from that of Si) in a certain amount to alleviate said lattice strain. As a result, the .beta. type zeolite and mordenite type zeolite synthesized had a Si/Al molar ratio of about 50 or less.
While zeolites differ in heat resistance and hydrothermal resistance depending upon the crystal structures, among zeolites of the same crystal structure, a zeolite of higher Al content has lower heat resistance and lower hydrothermal resistance.
Therefore, in order to allow a zeolite to have higher heat resistance and higher hydrothermal resistance, it is necessary to reduce the Al content in zeolite.
However, there has been no assurance that a zeolite having a pore structure suited for HC molecules to be adsorbed can be synthesized directly so as to have a low Al content at which desired heat resistance can be obtained.
Hence, when the Al content is higher than desired, it has been necessary to remove excessive Al (dealumination operation) by steam treatment, acid treatment or the like and repeat the operation until a desired Al content is obtained.